In case your Software Update hasn’t notified you already, there’s a Security Update waiting for your attention. It addresses a pretty long list of security concerns within OS X.
If you’d like to go and install it now – not a bad idea – then go up to the Apple menu and select Software Update. The security update will require a reboot, so be prepared for that.
The 2007-004 Security Update clocks in at just under 17mb and covers:
For more information you can check out the Apple docs page.